1. Field of the Invention
The present invention relates to a dielectric ceramic composition and a laminated ceramic capacitor.
2. Background Art
A laminated ceramic capacitor is generally produced through the following procedure.
Briefly, a dielectric material sheet is prepared and a material for forming an internal electrode is applied thereto. A composition predominantly comprising BaTiO.sub.3 may be used as the dielectric material. Subsequently, sheets of dielectric material to which the electrode material has been applied are laminated and compressed with heating, and the resultant substance is fired at 1250-1350.degree. C. in air to thereby obtain a laminated ceramic having internal electrodes. Finally, external electrodes are baked onto both sides of the laminated ceramic so that the external electrodes can be connected to the internal electrodes to thereby obtain a laminated ceramic capacitor.
Noble metal such as platinum, gold, palladium or silver-palladium alloy has been used for the material of an internal electrode of a laminated ceramic capacitor. Although each of these metals has excellent properties required of a electrode material, these metals are expensive and increase manufacturing cost. Therefore, a laminated capacitor in which nickel, a base metal, is used for internal electrodes has been proposed so as to reduce manufacturing cost, and nickel has increasingly been used.
As electronic devices have become smaller in size, cheaper in price and of higher performance, the laminated ceramic capacitor is also required to be cheaper, more insulation-resistant from voltage breakdown, more insulated, more reliable and of higher capacitance. In order to reduce the price of electronic devices, a laminated ceramic capacitor in which nickel is used for internal electrodes is advantageously used because of its low price. However, conventional dielectric ceramic materials were designed to be used in a weak electric field only and they involve a problem in that insulation resistance, dielectric strength and reliability are drastically decreased when they are used in a strong electric field. No known laminated ceramic capacitor whose internal electrodes are formed of nickel can be used in a strong electric field.
More specifically, Japanese Patent Publication (kokoku) No. 42588/1982 and Japanese Patent Application Laid-Open (kokai) No. 101459/1986 disclose dielectric materials which have a high dielectric constant. However, the obtained dielectric ceramics have large crystal grains, resulting in disadvantages such as low dielectric strength of a laminated ceramic capacitor under a strong electric field and a short life in a high-temperature test.
Japanese Patent Publication (kokoku) No. 14611/1986 and Japanese Patent Application Laid-Open (kokai) No. 272971/1995 disclose dielectric materials which have high dielectric constants of 2000 or more and 3000 or more under a weak electric field, respectively. However, these materials have several disadvantages, such as a considerably low dielectric constant (electrostatic capacity) and low insulation resistance under a strong electric field. These materials comprise Li.sub.2 O--SiO.sub.2 or B.sub.2 O.sub.3 --SiO.sub.2 oxide serving as an additive component and these oxides diffuse while firing, causing lot-to-lot variation in electrical properties of fired products.
Accordingly, an object of the present invention is to provide a dielectric ceramic composition which can constitute a dielectric ceramic layer of a laminated ceramic capacitor, which has high weather resistance such as durability under high-temperature or good resistance in a humid environment, and which also has the following electric properties: the product of insulation resistance and electrostatic capacity represented by C.times.R is as high as 5000 .OMEGA..multidot.F or more at room temperature and 200 .OMEGA..multidot.F or more at 150.degree. C. under a strong electric field of 10 kV/mm, insulation resistance exhibits low dependence on voltage, electrostatic capacity is highly stable with respect to a change of DC bias voltage, dielectric strength is high and the rate of change in electrostatic capacity with temperature satisfies the B characteristic specified by JIS specifications and X7R characteristic specified by EIA specifications. Another object of the present invention is to provide a laminated ceramic capacitor comprising an internal electrode constituted by Ni or Ni alloy and the aforementioned dielectric ceramic composition serving as a dielectric ceramic layer.